Mr.Smith is the only one of those foreigners who in ChinaA. workB. is workingC. are work

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A. work
(答案→)B. is working
C. are working
D. had working
解析：考查定语从句。本题定语从句的先行词是the only on，而不是those foreigners。关系代词who指代先行词the only one在句中做主语，后面使用单数的形式。句意：斯密斯先生是那些外国人中唯一的一个现在在中国工作的人。故B正确。D项的时态错误。